37^ iWr. Lee ow the dispersive power 
the result of which, and the manner of conducting them, I 
shall now take the liberty of laying before the Society. 
The first instrument employed for the purpose with any 
degree of satisfaction, was the two feet reflector made by Mr. 
Short, and which belongs to the Royal Society. In the com- 
pound focus of the eye piece of this telescope, I fixed horizon- 
tally a narrow slip of ivory. With the instrument thus prepared, 
I observed Capella, and other low stars near the meridian. By 
carefully noticing the intervals of time between the first con- 
tact and total immersion, and between the first appearance 
and complete emersion of the star from behind the slip of 
ivory, I obtained data from which it was easy to calculate its 
vertical breadth, which, compared by estimation with its 
horizontal breadth, gave the separation of the extreme rays 
of light. 
It was impossible, however, to remain long satisfied with 
such coarse measures, and not finding it convenient to go to 
much expense on this account, I applied to my friend Mr. 
Rennie for the loan of his seven feet reflector made by Dr. 
Herschel, to which I adapted a very excellent wire micro- 
meter made by Mr. Troughton ; and thus, by the kind 
assistance of my friends, I obtained instruments capable of 
measuring small angles to the fraction of a second of space. 
With this apparatus I repeatedly measured the diameter of 
Mars during his opposition in 1813. The Society's apart- 
ments being well situated for the purpose, I observed the 
planet as soon as he became visible over the buildings, until he 
attained his meridian altitude, which never exceeded 15 0 . 
With a power of 170 and upwards, the disk of the planet 
appeared much elongated, especially when near the horizon ; 
the upper limb was of a fine blue, the lower limb of a deep red. 
